How to calculate Derating Parallel Conductors and Voltage Drop at the same time

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Electricalhelp

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The incoming service is 400 AMP/208V/3P. The distance to the transformer is 200 feet. The conductor they want to use is Aluminum.

If I run 2 Parallel conductors in one 5" PVC conduit of 500KCMIL with 8 carry conductors I get 490 AMPS.

How do I calculate the voltage drop if there is 8 carry conductors?

Since the service is more than 100 feet, do I need at least a minimum of 500 AMP?
 
You have 2 sets of parallel conductors so if you use something like Southwire's VD app you would just input 2 when it asks for the number of sets. The derating is a separate issue. 350 amps * 2= 700 amps * 70% = 490 amps so that's good for 400 amps. Why are you calculating for 8 CCC's?
 
I am using 8 carry conductors since there are 4 carry conductors in each set. (There is a total of 2 Sets)

When I use Southwire VD with a VD of 3%, I calculating for the minimum conductor size for 400AMP at 200 Feet away with 2 Parallel sets. The wire size they give me was 300kcmil.

So should I use 500kcmil is there will be 8 Carry Conductors in 1 Conduit?
 
Electricalhelp said:
I am using 8 carry conductors since there are 4 carry conductors in each set. (There is a total of 2 Sets)
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It is rare for the neutral to need to be counted as a CCC, so usually that would be 6 CCCs.
Electricalhelp said:
When I use Southwire VD with a VD of 3%, I calculating for the minimum conductor size for 400AMP at 200 Feet away with 2 Parallel sets. The wire size they give me was 300kcmil.
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Then 300 kcmil is OK for voltage drop. Whether it is OK for ampacity is a separate check.
Electricalhelp said:
So should I use 500kcmil is there will be 8 Carry Conductors in 1 Conduit?
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If the logic behind jumping from 300 kcmil to 500 kcmil is the 80% (or 70%) derating factor as far as ampacity, that doesn't apply to VD. It's entirely separate, and there's no derating factors for voltage drop.

Cheers, Wayne
 
So what size conductor would I use, since I was wondering how I factor in the carry conductor and voltage drop of 200 feet?
 
Electricalhelp said:
So what size conductor would I use, since I was wondering how I factor in the carry conductor and voltage drop of 200 feet?
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(A) Ignoring length, come up with an NEC compliant choice of breaker, conductors, numbers of sets, etc that is sufficient for the NEC calculated load.
(B) Now determine or guess the real world current on the conductors.
(C) Check if that real world current at the actual length would have voltage drop exceeding your metric. If so, upsize the conductors as required. Go back to (A) and change any other choices as required for NEC compliance (e.g. EGC size).

Cheers, Wayne
 
Electricalhelp said:
So what size conductor would I use, since I was wondering how I factor in the carry conductor and voltage drop of 200 feet?
Click to expand...
In order to calculate voltage drop as Wayne stated you need to know the current on those conductors. You have a 400 amp service so for the worst case scenario use can 400 amps for your voltage drop calculation. As you stated in post #4, 2 sets of 300 kcmil aluminum are large enough for your voltage drop. If you don't count the neutral as a CCC then as Wayne suggested you used the 80% factor for derating so you end up with 260 amps * 2 = 520 * 80% = 416 amps.

So to summarize:
2 sets-300 kcmil VD=2.95% @ 400 amps
6 CCC's-80% derating
416 amps. (y)
 
If I run 2 Parallel conductors in one 5" PVC conduit of 500KCMIL with 8 carry conductors I get 490 AMPS.
Click to expand...
What do you mean you get 490 Amperes? Did you use Clamp meter to get this value or it is your calculated total current?
 
300 MCM aluminum according to NEC Table 9 the impedance it is 0.082 ohm/1000 ft for phase-to-neutral in PVC conduit. For thee phase a 1.73 factor has to be employed. If the voltage is 208/120 V then 200 ft and 400 A the voltage drop will be 0.082/2*0.2*1.73*400=5.68 V [5.68/208*100=2.73%]

On the other hand 300 MCM aluminum according to Table 310.15(B)(16) withstand 260 A at 90oC.

Using adjustment factor of 80% for up to 6 live conductors 260*2*0.8=416 A

According to art.110.14(C)(1)b 75oC it is permissible for 100 A and more. Then 260*0.8=208 < 230 A as permissible for 300 MCM aluminum at 75oC.
 
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